The present invention relates generally to the field of electromechanical pressure sensors and more particularly to the field of ceramic capacitive pressure transducers.
Ceramic capacitive pressure transducers are known and generally comprise parallel plate capacitor electrodes separated by an air gap wherein the spacing between the parallel plate electrodes is altered in response to a sensed pressure thereby changing the capacitance created by these electrodes. Generally, one capacitor electrode is deposited on a top end surface of a relatively thick cylindrically shaped ceramic base substrate while the other capacitor electrode is deposited on a relatively thin disc-shaped ceramic pressure sensing diaphragm. An annular glass insulating ring is deposited on a peripheral portion of the base substrate top surface and is used to bond the diaphragm to the base substrate as well as to space the diaphragm electrode a predetermined distance away from the base substrate electrode.
Typically, the diaphragm, the annular glass ring and the base substrate are assembled into a sandwich type structure and then heated to form an integral assembly such that the capacitor electrodes are spaced apart by a predetermined distance totally dependent upon the thickness of the annular glass ring. Generally, a vacuum entryway hole is provided through the base substrate, and through this hole a predetermined reference vacuum pressure is applied to an internal cavity formed by the diaphragm, the annular glass ring and the base substrate. Subsequently, the vacuum entryway is sealed so that the internal cavity will maintain (store) a predetermined reference vacuum pressure.
By applying various degrees of pressure to the exterior of the capacitive pressure transducer, the transducer diaphragm is flexed by predetermined amounts and this results in changing the capacitance created by the capacitor electrodes since the flexing of the diaphragm changes the spacing between the electrodes. Thus by monitoring the capacitance created by the electrodes, the ceramic capacitor transducer will produce an electrical signal related to the magnitude of the exterior pressure applied to the diaphragm as compared to the magnitude of the reference vacuum pressure. Such transducers are readily adaptable for sensing vacuum pressures generated by automobile internal combustion engines.
Typically, the ceramic capacitor transducer is mounted to a housing wall by means of a flexible rubber O-ring which spaces the flexible diaphragm from the housing wall so as to form an exterior cavity essentially bounded by the housing wall, the flexible diaphragm and the O-ring. A through hole opening is provided in the housing wall and through this opening various pressures are applied to the exterior cavity which result in flexing the diaphragm and thereby altering the capacitance created by the pressure transducer. Generally the sealing O-ring has been mounted directly on an annular peripheral portion of the flexible diaphragm.
Pressure transducers constructed and assembled according to the foregoing descriptions have often been found to produce erratic pressure sensing readings or readings which did not directly vary in response to the sensed pressure. The present invention determined that these erratic readings were due to the stress applied to the pressure sensing diaphragm because of the assembly of the transducer to the housing wall. The present invention eliminates these erratic pressure sensing readings by providing an improved pressure sensor which can be mounted to a housing wall to create a pressure transducer assembly without applying any significant stress to the flexible diaphragm.